Localized 4f Research Articles

Controlling 4f antiferromagnetic dynamics via itinerant electronic susceptibility

Optical manipulation of magnetism holds promise for future ultrafast spintronics, especially with lanthanides and their huge, localized 4f magnetic moments. These moments interact indirectly by spin polarizing the conduction electrons (the Ruderman-Kittel-Kasuya-Yosida exchange interaction), influenced by interatomic orbital overlap, and the conduction electron's susceptibility around the Fermi level. Here, we study this influence in a series of 4f antiferromagnets, GdT2Si2 (T = Co, Rh, Ir), using ultrafast resonant x-ray diffraction. We observe a twofold increase in the ultrafast intersublattice angular momentum transfer rate between the materials, originating from modifications in the conduction electron susceptibility, as confirmed by first-principles calculations. Published by the American Physical Society 2024

Investigation of optical properties of Ce and Eu-doped Gd2SiO5 insights from GGA + U calculations

Gadolinium silicate, (Gd2SiO5) co-doped with Ce and Eu has been found to exhibit enhanced luminescence efficiency, which makes it a promising material for use in scintillators and phosphors. It has excellent scintillation properties such as high density and high Zeff. In this study, we used density functional theory (DFT) calculations within the Wien2k software to investigate the effect of Ce and Eu concentration and native defects on the electronic structure and optical properties of Ce and Eu co-doped Gd2SiO5. We utilized the DFT + U method to treat the localized 4f electrons of Ce and Eu. Our results indicate that the electronic structure and optical properties of Ce and Eu co-doped Gd2SiO5 are significantly affected by the concentration of the dopants and presence of native defects. We found that increasing the concentration of Ce and Eu dopants leads to a shift in the bandgap to lower energies, resulting in enhanced absorption and emission spectra. Moreover, our calculations reveal that presence of oxygen vacancies and Gd interstitials can induce new defect levels in the bandgap, which may affect the luminescence properties of the material. Our study provides valuable insights into the atomic-level mechanisms that govern the luminescence properties of Ce and Eu co-doped Gd2SiO5 which can aid in the design and optimization of luminescent materials for various applications.

Anisotropic magnetic and quadrupolar H−T phase diagram of CeRh2As2

The tetragonal heavy-fermion compound CeRh2As2 has intriguing low-temperature symmetry-breaking phases whose nature is unclear. The unconventional superconducting phase is complemented by other normal-state phases which presumably involve ordering of 4f electron multipoles supported by the Kramers doublets split by the tetragonal crystal electric field (CEF). The most striking aspect is the pronounced anisotropic H−T phase boundary for in-plane and out-of plane field directions. Using a localized 4f CEF model we demonstrate that its essential features can be understood as the result of competing low-field easy-plane magnetic order and field-induced quadrupolar order of xy type. We present calculations based on coupled multipole random phase approximation response function approach as well as molecular field treatment in the ordered regime. We use an analytical approach for a reduced quasiquartet model and numerical calculations for the complete CEF level scheme. We discuss the quantum critical properties as function of multipolar control parameters and explain the origin of a pronounced ac anisotropy of the H−T phase diagram. Finally, the field and temperature evolution of multipolar order parameters is derived and the high-field phase diagram is predicted. Published by the American Physical Society 2024

The nematic susceptibility of the ferroquadrupolar metal TmAg2 measured via the elastocaloric effect

Elastocaloric measurements of the ferroquadrupolar/nematic rare-earth intermetallic TmAg2 are presented. TmAg2 undergoes a cooperative Jahn-Teller-like ferroquadrupolar phase transition at 5K, in which the Tm3+ ion’s local 4f electronic ground state doublet spontaneously splits and develops an electric quadrupole moment which breaks the rotational symmetry of the tetragonal lattice. The elastocaloric effect, which is the temperature change in the sample induced by adiabatic strains the sample experiences, is sensitive to quadrupolar fluctuations in the paranematic phase which couple to the induced strain. We show that elastocaloric measurements of this material reveal a Curie-Weiss like nematic susceptibility with a Weiss temperature of T* ≈ 2.7K, in agreement with previous elastic constant measurements. Furthermore, we establish that a magnetic field along the c-axis acts as an effective transverse field for the quadrupole moments.

Orbital-overlap-driven hybridization in 3d-transition metal perovskite oxides LaMO3 (M = Ti-Ni) and La2CuO4

The wide tunability of strongly correlated transition metal (TM) oxides stems from their complex electronic properties and the coupled degrees of freedom. Among the perovskite oxides family, LaMO3 (M = Ti-Ni) allows an M-dependent systematic study of the electronic structure within the same-structure-family motif. While most of the studies have been focusing on the 3d TMs and oxygen sites, the role of the rare-earth site has been far less explored. In this work, we use resonant inelastic X-ray scattering (RIXS) at the lanthanum N4,5 edges and density functional theory (DFT) to investigate the hybridization mechanisms in LaMO3. We link the spatial-overlap-driven hybridization to energetic-overlap-driven hybridization by comparing the RIXS chemical shifts and the DFT band widths. The scope is extended to highly covalent Ruddlesden-Popper perovskite La2CuO4 by intercalating lanthanum atoms to rock-salt layers. Our work evidences an observable contribution of localized lanthanum 5p and 4f orbitals in the band structure.

Antiferromagnetic-to-Ferromagnetic Transition in the GdNiSi1 –xAlx Compound

In this work, the electronic structure and magnetic properties of GdNiSi have been investigated within the framework of theoretical DFT + U method. The antiferromagnetic ordering of the Gd moments was obtained as the ground state, whereas Ni and Si have negligible magnetic moments. An antiferromagnetic-to-ferromagnetic transition was found when GdNiSi is doped with Al in our theoretical calculations. The antiferromagnetic-to-ferromagnetic transition takes place in the Gd sublattice, the magnetic moments at the Gd ions remain the same but the type of magnetic ordering changes. It was found that even at concentrations as low as \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$x = 0.1$$\\end{document}, the ferromagnetic ordering in the Gd sublattice is more favorable in total energy as the ground state which can be ascribed to the shortening of Gd–Gd distances and the oscillating behavior of the Ruderman–Kittel–Kasuya–Yosida interaction in the Al-doped compositions. The electronic structure demonstrates significant changes in the vicinity of the localized Gd 4f states that confirms the presence of the antiferromagnetic-to-ferromagnetic transition using our theoretical results for Al \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$x \\geqslant 0.1$$\\end{document} in good agreement with the previous experimental magnetic results.

Optical control of 4f orbital state in rare-earth metals.

A change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare-earth elements with strong magneto-crystalline anisotropy (MCA) are important ingredients for magnetic devices. Thus, control of their localized 4f magnetic moments and anisotropy is one major challenge in ultrafast spin physics. With time-resolved x-ray absorption and resonant inelastic scattering experiments, we show for Tb metal that 4f-electronic excitations out of the ground-state multiplet occur after optical pumping. These excitations are driven by inelastic 5d-4f-electron scattering, altering the 4f-orbital state and consequently the MCA with important implications for magnetization dynamics in 4f-metals and more general for the excitation of localized electronic states in correlated materials.

Strain-induced magnetic anisotropy of multi-domain epitaxial EuPd2 thin films

Europium intermetallic compounds show a variety of different ground states and anomalous physical properties due to the interactions between the localized 4f electrons and the delocalized electronic states. Europium is also the most reactive of the rare earth metals which might be the reason why very few works are concerned with the properties of Eu-based thin films. Here we address the low-temperature magnetic properties of ferromagnetic EuPd2 thin films prepared by molecular beam epitaxy. The epitaxial (111)-oriented thin films grow on MgO (100) with eight different domain orientations. We analyze the low-temperature magnetic hysteresis behavior by means of micromagnetic simulations taking the multi-domain morphology explicitly into account and quantify the magnetic crystal anisotropy contribution. By ab initio calculations we trace back the microscopic origin of the magnetic anisotropy to thin film-induced uniform biaxial strain.

'Low' faecal immunochemical test (FIT) colorectal cancer: a 4-year comparison of the Nottingham '4F' protocol with FIT10 in symptomatic patients.

The aim of this work was to evaluate colorectal cancer (CRC) outcomes after 'low' (sub-threshold) faecal immunochemical test (FIT) results in symptomatic patients tested in primary care. This work comprised a retrospective audit of 35 289 patients with FIT results who had consulted their general practitioner with lower gastrointestinal symptoms and had subsequent CRC diagnoses. The Rapid Colorectal Cancer Diagnosis pathway was introduced in November 2017 to allow incorporation of FIT into clinical practice. The local '4F' protocol combined FIT results with blood tests and digital rectal examination (DRE): FIT, full blood count, ferritin and finger [DRE]. The outcome used was detection rates of CRC, missed CRC and time to diagnosis in local 4F protocols for patients with a subthreshold faecal haemoglobin (fHb) result compared with thresholds of 10 and 20 μg Hb/g faeces. A single threshold of 10 μg Hb/g faeces identifies a population in whom the risk of CRC is 0.2%, but this would have missed 63 (10.5%) of 599 CRCs in this population. The Nottingham 4F protocol would have missed fewer CRCs [42 of 599 (7%)] despite using a threshold of 20 μg Hb/g faeces for patients with normal blood tests. Subthreshold FIT results in patients subsequently diagnosed with a palpable rectal tumour yielded the longest delays in diagnosis. A combination of FIT with blood results and DRE (the 4F protocol) reduced the risk of missed or delayed diagnosis. Further studies on the impact of such protocols on the diagnostic accuracy of FIT are expected. The value of adding blood tests to FIT may be restricted to specific parts of the fHb results spectrum.

Anomalies in the Dirac bands in the proximity of correlated electrons.

Dirac fermions, particles with zero rest mass, are observed in topological materials and are believed to play a key role in the exotic phenomena in fundamental science and the advancement of quantum technology. Most of the topological systems studied so far are weakly correlated systems and the study of their properties in the presence of electron correlation is an interesting emerging area of research, where the electron correlation is expected to enhance the effective mass of the particles. Here, we studied the properties of Dirac bands in a non-symmorphic layered Kondo lattice system, CeAgSb2, employing high-resolution angle-resolved photoemission spectroscopy and first-principles calculations. In addition to the Dirac cones due to non-symmorphic symmetry, this material hosts Dirac fermions in the squarenet layer in the proximity of a strongly correlated Ce layer exhibiting Kondo behavior. Experimental results reveal crossings of the highly dispersive linear bands at the Brillouin zone boundary due to non-symmorphic symmetry. In addition, there are anisotropic Dirac cones constituted by the squarenet Sb 5p states forming a diamond-shaped nodal line. These Dirac bands are linear in a wide energy range with an unusually high slope. Interestingly, near the local Ce 4f bands, these bands exhibit a change in the slope akin to the formation of a 'kink' observed in other materials due to electron-phonon coupling. The emergence of such exotic properties in proximity to strongly correlated electronic states has significant implications in the study of complex quantum materials including unconventional superconductors.

Ab initio-based determination of lanthanoid-radical exchange as visualised by inelastic neutron scattering.

Magnetic exchange coupling can modulate the slow magnetic relaxation in single-molecule magnets. Despite this, elucidation of exchange coupling remains a significant challenge for the lanthanoid(iii) ions, both experimentally and computationally. In this work, the crystal field splitting and 4f-π exchange coupling in the erbium-semiquinonate complex [ErTp2dbsq] (Er-dbsq; Tp- = hydro-tris(1-pyrazolyl)borate, dbsqH2 = 3,5-di-tert-butyl-1,2-semiquinone) have been determined by inelastic neutron scattering (INS), magnetometry, and CASSCF-SO ab initio calculations. A related complex with a diamagnetic ligand, [ErTp2trop] (Er-trop; tropH = tropolone), has been used as a model for the crystal field splitting in the absence of coupling. Magnetic and INS data indicate antiferromagnetic exchange for Er-dbsq with a coupling constant of Jex = -0.23 meV (-1.8 cm-1) (-2Jex formalism) and good agreement is found between theory and experiment, with the low energy magnetic and spectroscopic properties well modelled. Most notable is the ability of the ab initio modelling to reproduce the signature of interference between localised 4f states and delocalised π-radical states that is evident in the Q-dependence of the exchange excitation. This work highlights the power of combining INS with EPR and magnetometry for determination of ground state properties, as well as the enhanced capability of CASSCF-SO ab initio calculations and purposely developed ab initio-based theoretical models. We deliver an unprecedentedly detailed representation of the entangled character of 4f-π exchange states, which is obtained via an accurate image of the spin-orbital transition density between the 4f-π exchange coupled wavefunctions.

Pressure modulated charge transfer and phonon interactions drive phase transitions in uranium–aluminum laves phases

Lanthanide AB2 intermetallic compounds known as Laves phases have itinerant 3d and localized 4f electrons, which lead to interesting physical properties such as magnetic anisotropy and high Curie temperatures. Actinide Laves phases can display physical properties that are similarly intriguing. However, at reduced A–A spacing the C14 and C15 polytypes may exhibit larger wavefunction overlap for their 5f electron states and distinct characteristics for phases with more delocalized chemical bonding. The C36 polytype, on the other hand, is extraordinarily rare (<5% of known Laves phases). UAl2 is the only known actinide Laves phase to show a pressure-controllable C15 → C36 transition. Here, we apply first principles calculations to determine the origin of the C15 → C36 phase transition and reveal the differences between the corresponding properties of each phase. Pressure increases lead to bond compression–induced electron transfer from Al to U, which drives dynamic instability in the C15 phonon modes because of the uniform U–U bonding environment. The opposite phenomena is observed in C36: varied U–U bonding environments are vibronically more stable after charge transfer. We find that the interplay between charge transfer, chemical bonding, and phononic stability are central to predicting phase transitions and corresponding changes in physical properties for both C15 and C36 UAl2.

Intrinsic spin shielding effect in platinum-rare-earth alloy boosts oxygen reduction activity.

Oxygen reduction reactions (ORRs) involve a multistep proton-coupled electron process accompanied by the conversion of the apodictic spin configuration. Understanding the role of spin configurations of metals in the adsorption and desorption of oxygen intermediates during ORRs is critical for the design of efficient ORR catalysts. Herein, a platinum-rare-earth-metal-based alloy catalyst, Pt2Gd, is introduced to reveal the role of spin configurations in the catalytic activity of materials. The catalyst exhibits a unique intrinsic spin reconfiguration because of interactions between the Gd-4f and Pt-5d orbitals. The adsorption and desorption of the oxygen species are optimized by modifying the spin symmetry and electronic structures of the material for increased ORR efficiency. The Pt2Gd alloy exhibits a half-wave potential of 0.95V and a superior mass activity of 1.5 A·mgPt-1 in a 0.1M HClO4 electrolyte, as well as higher durability than conventional Pt/C catalysts. Theoretical calculations have proven that the spin shielding effect of Gd pairs increases the spin symmetry of Pt-5d orbitals and adsorption preferences toward spin-polarized intermediates to facilitate ORR. This work clarifies the impact of modulating the intrinsic spin state of Pt through the interaction with the local high spin 4f orbital electrons in rare-earth metals, with the aim of boosting the spin-related oxygen reduction reaction, thus fundamentally contributing to the understanding of new descriptors that control ORR activity.

Magnetic tuning of band topology evidenced by exotic quantum oscillations in the Dirac semimetal EuMnSb2

Interplay between magnetism and electronic band topology is of central current interest in topological matter research. We use quantum oscillations as powerful tool to probe the evolution of band topology in the Dirac semimetal EuMnSb$_2$. The Eu local 4f magnetic moments display different antiferromagnetic states below 25 K and a field-polarized phase above 16 T. Upon cooling from 65 K into the field-polarized state, an exotic temperature dependent shift of oscillation peaks arises, accompanied by the development of non-zero Berry phase and a huge unconventional splitting of the oscillations. Band-structure calculations confirm the change from trivial to non-trivial band topology induced by the ferromagnetic Eu state, classifying EuMnSb$_2$ as unique magnetic topological semimetal.

Site‐dependent correlation strength and occupation number of 5f electrons in alpha phase plutonium metal

AbstractIn order to quantify the site‐dependent correlation strengths in terms of the quasi‐particle weights and the occupation numbers of 5f electrons in alpha phase plutonium metal with eight crystallographically nonequivalent atomic sites Pun (n = 1–8), we perform a first principles calculation by using a many‐body method merging density functional theory with dynamical mean‐field theory plus the relativistic and correlation effects. The quasi‐particle weight, the electronic spectrum function, the hybridization function, as well as the occupation number of Pu 5f electrons all suggest that Pu1 and Pu8 atoms have the most itinerant and the most localized 5f electrons, respectively, while the other atoms Pun (n = 2–7) exhibit an intermediate correlation strength. The quasi‐particle weights demonstrate that only the jj or intermediate coupling scheme is more appropriate for Pu 5f electrons in comparison with the Russel–Saunders (LS) scheme. The electronic spectrum function and the occupation analysis establish that Pu 5f electrons simultaneously have dual itinerant and localized regimes with average 5f occupation numbers of n5f between 4.8994 and 4.9628, which are mainly composed of 5f4, 5f5, and 5f6 configurations, irrespective of different atomic sites. Finally, we also estimate the so‐called quasi‐particle band structure to directly compare with angle‐resolved photoemission spectrum.

The Local and Electronic Structure Study of LuxGd1−xVO4 (0 ≤ x ≤ 1) Solid Solution Nanocrystals

Rare-earth-doped mixed crystals have demonstrated tunable optical properties, and it is of great importance to study the structural characteristics of the mixed-crystal hosts. Herein, LuxGd1-xVO4 (0 ≤ x ≤ 1) solid solution nanocrystals were synthesized by a modified sol-gel method, with a pure crystalline phase and element composition. The X-ray diffraction (XRD) and Rietveld refinement results showed that LuxGd1-xVO4 nanocrystals are continuous solid solutions with a tetragonal zircon phase (space group I41/amd) and the lattice parameters strictly follow Vegard's law. The detailed local structures were studied by extended X-ray absorption fine structure (EXAFS) spectra, which revealed that the average bond length of Gd-O fluctuates and decreases, while the average bond length of Lu-O gradually decreases with the increase in Lu content. Furthermore, the binding energy differences of core levels indicate that the covalent V-O bond is relatively stable, while the ionicity of the Lu-O bond decreases with the increasing x value, and the ionicity of the Gd-O bond fluctuates with small amplitude. The valence band structures were further confirmed by the first-principles calculations, indicating that the valence band is contributed to by the O 2p nonbonding state, localized Gd 4f and Lu 4f states, and the hybridized states between the bonding O 2p and V 3d. The binding energies of the Lu core and the valence levels tend to decrease gradually with the increase in Lu content. This work provides insight into the structural features of mixed-crystal hosts, which have been developed in recent years to improve laser performance by providing different positions for active ions to obtain inhomogeneous broadening spectra.

Field-induced Lifshitz transition in the magnetic Weyl semimetal candidate PrAlSi

Lifshitz transition (LT) refers to an abrupt change in the electronic structure and Fermi surface and is associated to a variety of emergent quantum phenomena. Amongst the LTs observed in known materials, the field-induced LT has been rare and its origin remains elusive. To understand the origin of field-induced LT, it is important to extend the material basis beyond the usual setting of heavy fermion metals. Here, we report on a field-induced LT in PrAlSi, a magnetic Weyl semimetal candidate with localized 4f electrons, through a study of magnetotransport up to 55 T. The quantum oscillation analysis reveals that across a threshold field B* ≈ 14.5 T the oscillation frequency (F1 = 43 T) is replaced by two new frequencies (F2 = 62 T and F3 = 103 T). Strikingly, the LT occurs well below the quantum limit, with obvious temperature-dependent oscillation frequency and field-dependent cyclotron mass. Our work not only enriches the rare examples of field-induced LTs but also paves the way for further investigation of the interplay among topology, magnetism, and electronic correlation.

Multi-experimental determination of magnetic transition in Weyl semimetals RAlGe

The single crystals of Weyl semimetals or candicate RAlGe (R = La, Ce, Pr and Nd) have been synthesized by self-flux method and confirmed forming in noncentrosymmetric LaPtSi type. Compared with nonmagnetic LaAlGe, the configurations of localized 4f electrons strongly dominate the magnetic, thermal and electric behaviors of other RAlGe. The main critical temperatures for magnetic transition are robust around 5.1 K, 15.4 K and 5.2 K for CeAlGe, PrAlGe and NdAlGe. PrAlGe and NdAlGe have c easy axis with similar anisotropy, but the easy direction for CeAlGe lies in a axis below 12 K. Their λ-shaped specific heat reveal 2nd order phase transitions. The magnetic entropy for PrAlGe and NdAlGe approach to the values of Hund’s ground states at about 60 K and 50 K, while that for CeAlGe reaches 87 % of Rln2 at Tc stemming from a doublet ground state. It’s significant to combine multi-experiments and effective fittings to comparatively explore Weyl RAlGe family for the potential application in spintronics.

Excellent 5f-electron magnet of actinide atom decorated gh-C3N4 monolayer.

Atomic functionality of two-dimensional (2D) materials, typically with a controllable doping route for offering regular atomic arrangement as well as excellent magnetism, is crucial for both fundamental studies and spintronic applications. Here, the adsorptions of the 5f-electron actinide series (An = Ac-Am) on porous graphene-like carbon-nitride (gh-C3N4) layers are explored to determine their structural stabilities, electronic nature and magnetic properties using the combination of density functional theory (DFT) calculations, ab initio molecular dynamics (AIMD), Monte Carlo (MC) simulations and chemical bonding analyses. Our investigations reveal that each An atom can be individually adsorbed at the vacancy site of gh-C3N4 sheet with high energetic, thermal and dynamical stabilities, which are rooted in the major interactions of ionic An-N bonding as well as the minor interactions of covalent bonding of An-5f6d states with N-2s2p states. The delocalization of a very few 5f electrons is dependent on whether they occupy the suborbitals that are matching and conducive to hybridize with the ligand orbitals forming the 5f-2s2p covalent bonds. We propose that the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism plays a determining role for the inter-atomic 5f-5f magnetic exchange via the 6d electrons as the conduction electrons. Large magnetic moment and magnetic anisotropy energy (MAE) from the localized 5f electrons, together with the metallic characteristics owing to the delocalized 6d electrons, render these An-based 2D materials excellent metallic magnets, especially for the U@gh-C3N4 system with the modest magnetic moment of 0.6 μB, large MAE of 53 meV and high Curie temperature (TC) of 538 K.

Norm-Conserving 4f-in-Core Pseudopotentials and Basis Sets Optimized for Trivalent Lanthanides (Ln = Ce–Lu)

We present here a set of scalar-relativistic norm-conserving 4f-in-core pseudopotentials, together with complementary valence-shell Gaussian basis sets, for the lanthanide (Ln) series (Ce-Lu). The Goedecker, Teter, and Hutter (GTH) formalism is adopted with the generalized gradient approximation (GGA) for the exchange-correlation Perdew-Burke-Ernzerhof (PBE) functional. The 4f-in-core pseudopotentials are built through attributing 4f-subconfiguration 4fn (n = 1-14) for Ln (Ln = Ce-Lu) into the atomic core region, making it possible to circumvent the difficulty of the description of the open 4fn valence shell. A wide variety of computational benchmarks and tests have been carried out on lanthanide systems including Ln3+-containing molecular complexes, aqueous solutions, and bulk solids to validate the accuracy, reliability, and efficiency of the optimized 4f-in-core GTH pseudopotentials and basis sets. The 4f-in-core GTH pseudopotentials successfully replicate the main features of lanthanide structural chemistry and reaction energetics, particularly for nonredox reactions. The chemical bonding features and solvation shells, hydrolysis energetics, acidity constants, and solid-state properties of selected lanthanide systems are also discussed in detail by utilizing these new 4f-in-core GTH pseudopotentials. This work bridges the idea of keeping highly localized 4f electrons in the atomic core and efficient pseudopotential formalism of GTH, thus providing a highly efficient approach for studying lanthanide chemistry in multi-scale modeling of constituent-wise and structurally complicated systems, including electronic structures of the condensed phase and first-principles molecular dynamics simulations.